Quick answer: Mulch Types and Benefits — Mulch is the cheapest thing that happens to a tree: a flat layer over the root zone holds moisture through the dry season, keeps the soil temperature steady and feeds the soil life the tree depends on. Almost every mulch problem is a depth problem or a contact problem — too thick, or piled against the trunk where the bark has to stay dry.

Typical depth: Two to four inches, flat, from the drip line inward — never a cone against the trunk.
Typical timing: After planting or after a clean-up; refreshed in autumn so winter rain carries it down.
What it involves: Keeping the root flare visible, keeping mulch off the bark, and choosing material that breaks down rather than mats.
The standard: ISA and university extension guidance on arboricultural mulching and the root flare.
Our own record: 503 pages and 291 stories published on this site; Uncategorized (109) is the rail this page files under. Counted from our own archive, not a stock table.

Field note. What we see most on this coast is not too little mulch but too much, in the wrong place: the volcano against the trunk that keeps the bark wet through the fog season and invites rot where the tree cannot show it until the canopy thins.

On the other hand, a thin ring of the wrong material is worse than none at all — matted, water-repellent mulch sheds the rain it was meant to store.

Standards and sources: Arboricultural standards (ISA — International Society of Arboriculture) · Soil and root health (UC IPM — University of California pest management)
Prepared by Cambria Pines Tree Service Inc. — ISA-certified arborists working on the Central Coast.

Mulch is not a category, it is a list, and the entries on the list behave differently enough that choosing the wrong one is a mistake you live with for years. The differences come down to three properties: particle size, carbon-to-nitrogen ratio, and how fast the material breaks down.

Arborist chip

The whole tree, chipped: leaves, twigs, bark and wood together. That mixture is its virtue. The leafy fraction breaks down fast and feeds the soil; the woody fraction persists and keeps the layer open. Carbon-to-nitrogen is high — around 100 to 1 — which is why it should not be dug in, and why on the surface it is close to ideal.

It is also the cheapest material on this list and the one most often given away.

Bark nuggets

Slow to break down, handsome, expensive, and poor at holding moisture because the pieces are large and shed water off their surface rather than into the layer. Use it where appearance matters and moisture is not the problem. Do not use it where you are trying to keep a bed cool.

Straw

Low density, high carbon, excellent at reflecting heat and at keeping fruit clean. It is the classic strawberry and garlic mulch. It is also the material most attractive to slugs, most likely to blow away, and most likely to come with seed in it.

Leaf mould

Fully decomposed leaves, and a different substance from leaves. It is low in nutrients, high in fungal biomass, and structurally excellent as a seed-raising medium and a soil conditioner. It is a mulch in the sense that it covers the ground, and a soil amendment in the sense that it is meant to be incorporated.

Compost

A mulch only at low depth. An inch of compost on the surface is a feed; three inches is a layer that seals, sheds water and stops gas exchange. Use it thin, and use it where the goal is nutrition rather than suppression.

Gravel

Permanent, non-decomposing, and the best of the lot at moving heat into the soil at night and holding it. It is also the best at shedding salt and the worst at being removed. Excellent for coastal beds, Mediterranean plantings, and around succulents; wrong for anything that wants an acid surface.

Wood shavings and sawdust

High carbon, fine particle, and prone to matting into an impermeable crust. As a surface layer over a small area it is acceptable. As a bed-wide mulch it is a problem, and as a dug-in amendment it is a nitrogen robbery.

Rubber crumb

It does not decompose, it does not feed anything, and it leaches zinc and other compounds into the soil over time. Its one genuine use is impact attenuation on a playground, and that is a safety specification rather than a horticultural one.

The ratio that explains most of it

Materials with a high carbon-to-nitrogen ratio — chip, straw, sawdust, bark — are broken down by microbes that need nitrogen, and they take it from the soil around them if the material is incorporated. On the surface, that competition happens at the boundary and the effect is small. Dug in, it is severe and it lasts a season.

Materials with a low ratio — compost, leaf mould — release nutrients rather than lock them, which is why they cannot be used thick.

Choosing

Ask what the mulch is supposed to stop. Heat, moisture loss, weeds, salt, mud, or appearance — pick the one that matters most and the material follows. The decision order is set out in choosing a mulch by the question it answers.